Technique for measuring overlay between layers of a multilayer structure

ABSTRACT

A method for determining overlay between layers of a multilayer structure may include obtaining a given image representing the multilayer structure, obtaining expected images for layers of the multilayer structure, providing a combined expected image of the multilayer structure as a combination of the expected images of said layers, performing registration of the given image against the combined expected image, and providing segmentation of the given image, thereby producing a segmented image, and maps of the layers of said multilayered structure. The method may further include determining overlay between any two selected layers of the multilayer structure by processing the maps of the two selected layers together with the expected images of said two selected layers.

FIELD OF THE INVENTION

The present invention relates to a technique for determining overlaybetween layers of a multilayer structure by analyzing an image whichrepresents such a structure. For example, the invention relates toautomated inspection of modern integrated circuits which constitutemicro-miniature semiconductor structures with multiple layers producedby lithography. More specifically, the invention relates to overlayassessment techniques which require utilizing a Scanning ElectronMicroscope (SEM) for obtaining the representing image.

BACKGROUND OF THE INVENTION

Many technological fields may suggest examples of complex multilayerphysical objects. Internal structure of such objects may be studied byanalyzing available images of the objects. Examples of such analysis maybe found in inspection of modern 3D semiconductor structures, as well asin geophysics, biology, medicine, in medical equipment technologies suchas Computer Tomography, etc. The images to be analyzed may be obtainedby various technologies utilized in the corresponding fields.

The present patent application will describe the mentioned techniqueusing an example from the field of inspection of modern multilayersemiconductor structures.

The modern multilayer semiconductor structures of interest have arrivedto such a scale of miniaturization (presently, up to nodes scale ofabout 7-10 nm) that they cannot be inspected with required accuracy andresolution by optical microscopes, since information provided by opticalmicroscopes is a result of processing of visual images. For such modernstructures, there is a theoretical option to apply complex, model-basedanalysis methods for processing data obtained by visual measurements.

An alternative, more practical option is to utilize technologiesinvolving tools having resolution higher than that in opticalmicroscopes.

Such tools, for example scanning electron microscopes (SEMs), are oftenused in inspection of semiconductor wafers. SEMs may be used to detectand classify defects in production of microelectronic devices, toprovide sophisticated process control, etc. SEM images, however, containa wealth of detail, which must be properly interpreted in order toidentify the structures appearing in each image, to distinguish thestructures from other features and to estimate their relativecoordinates.

In order to proceed with the description, some comprehensive definitionshave been introduced below, which are important for understanding theproblem and the exemplary solutions which will be described below.

Three-dimensional integrated circuit (3D IC)—an integrated circuitmanufactured by stacking silicon wafers and/or dies and interconnectingthem vertically using through-silicon vias (TSVs) so that they behave asa single device. In the present description, we speak about 3D ICmanufactured using fab processes by gradually depositing multiple layers(dies) one onto another. 3D IC is one preferred example of a Multilayerstructure.

SEM—Scanning Electron Microscope used for exposing a 3D IC to a primaryelectron beam , collecting data on responsive electron beams orscattering electrons from multiple layers of the 3D IC and furtherreconstructing a combined SEM image of the multiple layers by applyingcomputer processing to the collected data.

Available image or Given image—an image representing a real multilayerstructure; SEM-image—an example of Available/Given image.

CD SEM—Critical Dimensions Scanning Electron Microscope, which isapplicable in a wide range of nodes having dimensions from about 3000 nmto about 7 nm. CD-SEM delivers High Resolution, High Throughput, HighSensitivity and High Repeatability by utilizing more sophisticatedelectron optics and advanced image processing.

Expected image—image of one or more specific details/features/structuresto be constructed and/or to be found in a specific layer of amultilayered structure. For example, the Expected image may be a designimage, for example a CAD-image created by utilizing CAD (computer-aideddesign) tools for designing features of a specific layer. Expected imagemay be a design image, simulated so as to look closer to a real objectafter manufacturing thereof according to the design.

A combined Expected image—image obtained by combining Expected images oflayers of the multilayer structure, taking into account visibility ofthe layers and elements thereof.

Overlay (OVL)—vector characterizing pattern-to-pattern alignment of onelayer of a multilayer structure with respect to another layer thereof.

Modern silicon wafers are currently manufactured in a sequence of steps,each stage placing a pattern of material on the wafer; in this waytransistors, contacts, etc., all made of different materials, are laiddown. In order for the final device to function correctly, the separatepatterns of the layers must be aligned correctly. Overlay control iscontrol of the above-mentioned pattern-to-pattern alignment.

Registration—maximally possible alignment of two or more images byrecognition of their mutual positioning. For example, registration maybe performed by achieving maximal possible overlapping between theimages. Various technologies of registration exist; a version ofregistration, customized for advanced semiconductor nodes will beproposed in the present description.

Segmentation of available image—labeling pixels of the available imageto associate the pixels with different classes of features (objects,elements). The features may be located at different layers of themultilayer structure. Image segmentation is typically used to locateobjects and boundaries (lines, curves, etc.) in images. More precisely,image segmentation is the process of assigning a label to every pixel inan image such that pixels with the same label share certaincharacteristics. For example, a specific label may indicate a specificlayer of the structure.

Segmentation of SEM-image—labeling pixels of a SEM-image of a multilayerstructure, in order to obtain a segmented image Segm(x,y) wheresimilarly labeled pixels form segments. The Segmentation is a process oflabeling pixels of the SEM image by assigning, to every pixel havingcoordinates(x,y), an index j being index of the layer to which thispixels belongs. The index j may accept values from the interval {1 . . .N} where N is a number of layers of the multilayer structure.

Maps of layers of an N-layer multilayer structure—is a set of N binaryimages each having the size of the given image.

Map of a specific layer is a binary image comprising features of thatlayer only, which are visible on the Given image (and consequently onthe Segmented image), and which are represented on the Map by areas ofpixels (segments) labeled with the label of that specific layer. It maybe written down as follows:

Map (x, y){j}={1, if Segm (x,y)=j; 0 if Segm(x, y)≠j, 1<=j<=N, where Nis a number of layers}.

The terms “area of pixels” and “segment” will be used intermittently inthe description.

There is a long felt need in solutions for effective and accurateestimation of overlay in multilayer structures, for example such asmodern micro miniaturized multilayer semiconductor structures.

SUMMARY OF THE INVENTION

One of the objects of the invention is providing a more universaltechnique for analysis of an image representing a real multilayerstructure in order to determine overlays between layers of thestructure.

A more specific object of the invention is providing an effective andaccurate technique for overlay measurement in multilayer semiconductorstructures. That object has been recognized by the Inventors as a longfelt need for a centralized, global measurement of overlay in the modemsemiconductor multilayer structures.

Such effective and accurate solutions are expected to allow reliableestimation of overlays/shifts in the multilayer structure, to minimallyaffect the desired throughput during the fab process, and to enableon-line adjustment of the fab process.

According to a first aspect of the invention, there is provided a methodfor determining overlay between layers of a multilayer structure, themethod comprising

-   -   providing an image (a so-called given image or available image)        representing the multilayer structure,    -   obtaining expected images for respective layers of the        multilayer structure;    -   providing a combined expected image of the multilayer structure        (as a combination of the expected images of said layers, taking        into account an order of the layers and visibility/expected        occlusions due to placement of the layers one onto another);    -   performing registration of the given image against the combined        expected image;    -   providing segmentation of the given image, thereby producing        -   a segmented image, (wherein each pixel of the segmented            image is associated with a label indicating a layer of the            multilayer structure to which the pixel is related, thereby            creating areas/segments of pixels having identical labels),            and        -   segmentation maps, also called maps of the layers of said            multilayered structure (wherein such maps is a set of binary            images, and wherein a specific layer's map is a binary image            comprising features of that layer only, which are visible on            the given image (and therefore on the Segmented image) and            which are represented on the map by segments labeled with            the label of that specific layer);    -   determining (measuring) overlay between any two selected layers        of the multilayer structure by processing the maps of the two        selected layers together with the expected images of said two        selected layers.

Advantageously, the proposed method allows determining overlay for eachlayer of the multilayered structure, with respect to any of theremaining layers. In practice, overlay may be determined for all layersof the structure, in any combinations.

It should be noted, that the expected images of the layers and/or thecombined expected image may be understood as design images which haveundergone simulation, for creating similarity thereof to the given imageof the real multilayer structure (i.e., to imitate real appearance ofthe combined image/layers upon fabrication). Such expected images arecalled simulated design images.

According to one version of the method, it is a method of measuringoverhead between layers of a multilayered semiconductor structure,wherein

-   -   the multilayer structure is a semiconductor structure such as 3D        IC, for example a wafer,    -   the given image is a SEM-image of the 3D IC,    -   the expected images are simulated design images (for example,        CAD-images of the layers, simulated so as to imitate real        appearance of the layers upon fabrication),    -   the combined expected image (for example, a combined CAD-image)        is formed by combining the mentioned design images of the layers        taking into account the order of the layers and the expected        occlusions;    -   the segmentation is a SEM-image segmentation, thereby producing        a segmented image (SEM-image) of the structure, and also        separate maps (SEM-maps) of the layers,    -   the overlay between any two layers of the 3D IC is determined by        processing the maps of the two layers together with the        expected, preferably simulated images of said two layers (for        example, by processing the SEM-maps of these layers using the        simulated CAD-images of the same layers).

It should be noted, that the segmentation may refer to the expectedimages of the layers, i.e. the method may comprise taking the expectedimages into account while performing the segmentation.

The proposed technique may be improved by iteratively enhancing thesegmentation, so as to more accurately distinguish features located ondifferent layers or on the same layer, per pixel of the given image(SEM-image).

Results of the segmentation may be corrected by adjusting the expectedimage(s) of the layer(s). It can be done based on the measured overlay.

Therefore, the segmentation results may be enhanced by using feedbackabout the measured overlay.

It should be noted that a different technique intended for measuringoverlay and using segmentation may also be improved by iterativelyenhancing the segmentation based on results of the overlay measurements.

Consequently, upon correcting the segmentation results, results of theoverlay measurement will also be improved.

In view of the above, the method may comprise correcting thesegmentation results by:

-   -   correcting the expected image of a specific layer based on the        overlay value measured relative to the specific layer, (for        example, by changing coordinate(s) of the expected image by        shifting),    -   correcting the segmentation by taking into account the corrected        expected image, for example by obtaining a corrected map of the        specific layer, based on the corrected expected image of said        specific layer;

The overlay may be then measured again, based on the correctedsegmentation, and the method may be continued iteratively.

The mentioned feedback, and more specifically the step of correcting theexpected image (CAD-image) of a layer may be caused, for example by asituation where the measured overlay does not exceed a predeterminedlimit of overlay and/or can be improved by shifting/adjusting CAD-imageof the layer to be closer to the corresponding map (SEM-map) of thelayer, thus improving the segmentation results on the next step of theprocess.

The method may comprise as many iterations as allowed by a set ofexisting/predetermined limitations. The limitations may relate to time,quality and cost of the overlay measurement procedure, overlay value,etc. Some other limitations will be mentioned as the descriptionproceeds.

For more effective overlay measurement, the method may comprise takinginto account additional, probable occlusions which could be causedwithin one or more predetermined limits of acceptable offset.

Such limits may be known in advance for manufacturing multilayerstructures (such as semiconductor wafers) that satisfy specific qualityrequirements. One example of the discussed limits may be “CAD to SEMmaximal variation” which limits acceptable changes of elements' sizes ona layer. Another example of the limits is “CAD to SEM maximal shift”,which limits acceptable overlay between specific layers. For example,elements of one transistor, which are deposited on different layers ofthe structure, will not form the properly operating transistor ifoverlays between these different layers and/or sizes of the elementsvary over a set of some predetermined accepted limits Structures wherethe overlay exceeds the predetermined limit/s are usually considered asdefective. In some cases, when CAD correction is performed (say, thelayer's CAD image is shifted to become closer to the segmented image ofthe layer), the segmentation could be improved through more confidentuse of CAD information.

The additional/probable occlusions may be taken into account at the stepof segmentation and even thereafter, to define so called safe areas onthe expected images of the layers and respectively on the maps of thelayers, said safe areas being those associated with such segments ofelements belonging to said layer, which segments would be visible on thegiven image despite the expected and the additional (probable)occlusions and distortions between the expected images and the givenimage. An example of such a distortion may be a CAD-SEM imagesdeviation.

In other words, the safe areas relate to segments of the elements, whichremain visible i.e., cannot be occluded by any limited offset ordistortion (maximal possible overlay, probable variations of sizes,etc.). It goes without saying that elements which are not occluded atall will be considered to form safe areas on the expected images and onthe maps of the layers.

The use of segmentation results referring to the safe areas would beconsidered more effective/reliable.

The step of determining (measuring) of overlay between any two layers ofthe multilayer structure may be performed, for example, according to oneof the following two versions.

In a first version of measuring overlay, the method may comprise:

-   -   for each of the two selected layers, performing per-layer        registration between the map (a SEM-map) of the specific layer        with the expected image (a design/CAD-image) of the same        specific layer, by referring to the safe areas;    -   for each of the two selected layers, measuring a shift (or        vector of registration) of a specific layer by comparing the map        (a SEM-map) of the specific layer with the expected image (a        design/CAD-image) of the same specific layer, thereby obtaining        two shifts of the two selected layers respectively;        obtaining the overlay between said two selected layers as a        difference between said two shifts.

The measurement of the shift (the vector of registration) may bepresented as measuring of X/Y shift of the map (say, SEM map) of thespecific layer along two axes X and Y with respect to the expected image(say, CAD-image) of said specific layer;

For example,X-component of the shift of layer “i ” along axis X may be determinedclose to the equation 1:

ΔXi=XSEMi−XCADi   [1]

X-component of the overlay of layer “ i ” in respect of layer “ j” canbe determined close to the equation 2:

ΔXi−ΔXj=XSEMi−XSEMj+(XCADj−XCADi)=XSEMi−XSEMj+offset,   [2]

where (XCADj−XCADi)=offset=constThe analogous operations are performed for axis Y, to obtain Y-componentof the overlay.

It should be noted that said per-layer registration may be performedmore effectively if the registration is based on the safe areasdescribed above.

According to a second option of measuring the overlay, the method maycomprise:

-   -   choosing two safe areas (MSA) respectively appearing in the two        selected maps of the two selected layers;    -   allocating two segments (ESA) respectively appearing in the two        expected images of the two selected layers, said two segments        ESA respectively corresponding to said two areas MSA;    -   determining a vector V1 (vector of overlay S_(img)) of overlay        for the two chosen safe areas MSA by calculating a difference        between Centers of Gravity (COG) thereof;    -   determining a vector V2 (vector of overlay S_(exp)) for the two        segments ESA by calculating a difference between Centers of        Gravity (COG) thereof;    -   determining overlay between the selected two layers by        calculating a difference between V1 and V2.

For inspection of semiconductor structures, V1 is preferably estimatedon the SEM image, while V2 is estimated on the CAD image.

The second option of the overlay measurement may be performed when onlyone of said two areas is MSA and/or only one of said two segments isESA, while the other areas/segments are not the safe areas.

Again, the method is advantageous since it is adapted to performmeasurement of overlay for any number of additionally selectedalternative pairs of the layers, in other words—for all layers of themultilayer structure.

The inventive method may alternatively be defined as follows:

A method for performing determining overlay between layers of amultilayer structure, the method comprising

-   -   obtaining a given image representing the multilayer structure,    -   obtaining expected images for respective layers of the        multilayer structure and a combined expected image of the        multilayer structure;    -   performing registration of the given image against the combined        expected image;    -   performing segmentation of the given image;    -   determining overlay between any two selected layers of the        multilayer structure,    -   correcting results of the segmentation using feedback about the        determined overlay.        The method may further comprise re-determining overlay using the        corrected segmentation results.

Yet another independent definition of the inventive method may be asfollows:

A method for determining overlay between layers of a multilayerstructure, the method comprising

-   -   obtaining a given image representing the multilayer structure,    -   obtaining expected images for respective layers of the        multilayer structure and a combined expected image of the        multilayer structure;    -   performing registration of the given image against the combined        expected image;    -   providing segmentation of the given image, thereby producing        -   a segmented image, and        -   maps of the layers of said multilayered structure;    -   determining safe areas on the expected images and on the maps of        the layers, wherein a safe area representing a segment of an        element on a specific layer, which segment cannot be occluded by        any predetermined offset;    -   determining overlay between any two selected layers of the        multilayer structure by processing the maps of the two selected        layers together with the expected images of said two selected        layers, by utilizing the determined safe areas.

It should be kept in mind that the method described above isadvantageous for inspecting the multilayer structures such as modemsemiconductor wafers or 3D IC, based on SEM-imaging of the structure andpreliminarily designed images of the layers. Moreover, it easily allowsmeasuring overlay values for all layers of the structure.

During fabrication of a 3D integrated circuit, upper layers of the 3D ICmay still be removed if they introduce a critical overlay. The processmay be continued to replace the removed layers with new ones, probablyusing corrected CAD-images for the corresponding layers. When overlayvalues can be corrected by correcting CAD-images, this may be performedby providing a high-order Stepper (scanner) correction before productionof a next wafer.

The scanner is able to process up high order correction along each axis,for example (a+bx+cx̂2+dx̂3), where x is an overlay between two specificlayers. Using CD-SEM overlay results and some overlay mathematicalmodel, a user/customer can find those coefficients a, b, c, d and usethem in the scanner to fix the aberrations.

The newly proposed method is effective for any structures and especiallyadvantageous for 3C IC structures inspected using SEM, since it is

generic, i.e. does not depend on geometry of layers to be inspected,global, i.e. allows performing measurements between layers and notbetween elements in the layers;comprehensive/universal, since the information is obtained for alllayers of the structure simultaneously (in parallel); the informationallows computing overlay for any pair of the layers;allows feedback for improving accuracy by iteratively improvingsegmentation of the available image.robust, since it is stable to noise or to low signal to noise (SNR)ratio,accurate, by providing measurements with sub-nm accuracy.

According to a second aspect of the invention, there is also provided asystem for image processing, including a memory, which is configured tostore a given image (for example, SEM-image) of a multilayer structure(for example, 3D IC) and expected images of the layers. In one example,the expected images are design (CAD) images of the layers used infabricating the 3D IC; in another example they are design imagessimulated to suit the given image. The system also comprises a processorconfigured

-   -   to generate a combined expected image (optionally, with        preliminarily simulating the design images),    -   to process the given image with the combined expected image so        as to perform registration there-between,    -   to perform segmentation of the given image into maps of the        corresponding multiple layers of the structure, and    -   to perform overlay measurement between any two layers of the        structure, based on the maps and suitable expected images of        said two layers.

There is additionally provided, in accordance with an embodiment of thepresent invention, a computer software product, including anon-transitory computer-readable medium in which program instructionsare stored, which instructions, when read by a computer, cause thecomputer to perform steps of the method described above.

The invention will further be described as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with the aid of thefollowing non-limiting drawings in which:

FIG. 1—schematically illustrates an embodiment of the system adapted toperform the inventive method for measuring overlay in a multilayerstructure. In FIG. 1, the structure is a semiconductor wafer, theinitial (given) image is obtained by SEM and processed by a computerprocessor.

FIG. 2 illustrates a schematic block diagram of one version of theproposed method.

FIG. 3 illustrates a block diagram of another version of the proposedmethod, which comprises determining areas safe from occlusion onexpected images of the layers and on their maps, which safe areas allowmore effective segmentation and, consequently, more accurate measurementof overlay.

FIG. 4 illustrates a block diagram of a further version of the proposedmethod, which comprises feedback that allows improving overlay atfurther steps of manufacturing/inspection based on the obtainedmeasurements.

FIG. 5 illustrates a block diagram of one exemplary process (using perlayer registration) for measuring overlay between two selected layers ofthe multilayer structure.

FIG. 6 illustrates a block diagram of another exemplary process (usingCOGs approach based on safe areas) for measuring overlay between twoselected layers of the multilayer structure.

FIG. 7 presents a schematic illustration of different stages of theproposed process by pictorial illustrations of: a combined expectedimage and a given image of a structure; an expected image and a map ofone layer; stages of defining so-called safe areas.

FIG. 8 is a schematic illustration of the exemplary process shown inFIG. 6.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is a pictorial presentation of an exemplary set of equipment forimplementing one embodiment of a system S according to the invention. Inthe example shown in FIG. 1, the system S is intended for inspection ofmultilayer semiconductor structures (3D IC, wafers) using an electronscanning microscope SEM.

The system S comprises a computer C with the processor (not shown) andthe memory which is shown schematically as an external block M. Thecomputer is also equipped with a display D and a keyboard K so that anoperator may control and adjust the inspection process. The computer Cis in communication, via line L, with the scanning electron microscopeSEM which is adapted to create an image of a three-dimensionalmultilayer structure (a semiconductor wafer W is shown).

The SEM-image obtained in the SEM is transmitted to the computer C,where it is processed and stored in the memory M.

Memory M of the computer C also can store a set of expected imagespreliminarily developed for layers of the wafer W.

Operation of the system S is focused on measuring overlay between layersof the wafer, based on processing the given image (SEM-image) and theexpected images.

The expected images may be CAD images designed for the layers.Alternatively, the expected images may be formed from the design imagesupon simulation, in order to make them looking maximally similar to realpatterns on layers of the real structure of interest.

In the present example, the computer software may comprise one or moreprograms for simulation of the design images so as to convert them intoexpected images maximally close to images which could be obtained uponmanufacturing the layers and scanning them by the SEM. The specificembodiment of the proposed system, intended for measuring overlay insemiconductor wafers, operates more effectively with such a simulation.The computer accommodates some computer readable media which containsthe proposed software product (schematically shown as a dotted contourSP) responsible for novel functions of the system. Some of the functionswill be described with reference to flowcharts illustrated in thefollowing drawings.

FIG. 2 shows a flow chart of one version of the proposed method. Theversion 10 may serve a basic one for some modifications of the method.

Block 12: obtaining a given image of a real multilayer structure. In aspecific example described in the present application, the given imageis a SEM-image generated by a scanning electron microscope.

Block 14: obtaining expected images of layers of the multilayerstructure.

-   -   Box 14.1 indicates a plurality of design images developed for        multiple layers of the structure, including an exemplary layer        “i”.    -   Block 14 may comprise Simulation of the design images, as        follows:    -   Box 14.2 indicates that the design images may be simulated to        respectively form expected images of the multiple layers;    -   Box 14.3 indicates that a combined expected image may be        obtained from the simulated images of the layers.

The expected images may be, for example, CAD images or simulated CADimages.

The mentioned variations of the expected images are stored in thecomputer memory.

Further, the given image may be compared with the expected images, usingthe following operations.

Block 16 indicates that the given image (SEM-image) is registered withrespect of the combined expected image.

Block 18 is responsible for Segmentation of the given image, namely:

Box 18.1 denotes that a segmented image is obtained, which means thateach pixel of the given image is “labeled” by a label indicating thelayer to which the specific pixel is related. The segmented image isstored in the computer memory.

Box 18.2 indicates that a plurality of layer maps (including the map ofan exemplary layer “i”) are then formed from the segmented image andstored in the memory. In our specific example, the maps are SEM-maps ofthe respective layers.

Optionally, the segmentation process which is performed in boxes 18.1and 18.2 may be assisted and facilitated by taking into account thesuitable expected images received in block 14 (these optional arrows arenot shown in FIG. 2).

Block 20 is a block of overlay (OVL) measurement. The concept ofmeasuring overlay proposed by Block 20 is the basis of a so-calledglobal overlay measurement. Namely, OVL between any two layers in amultilayered structure can be measured by processing their expectedimages and their maps together. If in our specific example the expectedimages are CAD-images, overlay between any two selected layers can bemeasured by processing the CAD-images and the SEM-maps of the selectedtwo layers.

Some specific implementations and combinations of the concepts definedin the flowchart 10 will be described with reference to the followingdrawings.

FIG. 3 shows a fragment of a flowchart, illustrating how an additionalfeature, namely so-called safe areas, may be determined and combinedwith the method shown in FIG. 2.

It should be noted, however, that the concept of safe areas may be usedalso with other methods for inspection of semiconductor structures, notonly with the methods of overlay measurement described in this patentapplication.

The safe areas are defined in a new Block 15.

Block 15 comprises defining safe areas for each layer “i”, first on theexpected image of that layer (in our specific example, on the simulatedCAD-image of the layer). The safe area should be understood as such anarea of an element/feature of the specific layer, which area cannot beoccluded by any limited offset. In other words, a safe area of a featureof a specific layer should remain visible (i.e. not occluded by featuresbelonging to other layers) at maximal allowed deviations of thefeatures' sizes and of X/Y overlay between layers. There arepredetermined limits of those deviations. The deviation limits areschematically indicated by an arrow of data fed to the block 15. Thedefined safe areas are usually smaller than the expected visual segmentsof the features, i.e. than those considered visible in the expectedimages of the layers.

(FIGS. 7 and 8 will further provide some pictorial illustrations of thesafe areas' meaning.)

Block 15 defines such safe areas on an expected image (say, a CAD image)of a specific layer. Let these safe areas be called ESA (expected safeareas). How it is done? A pixel of the expected image, which wasdesigned as visible and still remains visible at any alloweddeviations—will be considered to belong to an ESA. The similar operationis performed for each layer of the structure.

Optionally, the expected images of the layers, with the defined safeareas ESA, may be used for obtaining the combined expected image (block14.3, not shown in FIG. 3) and then for proper registration in box 16.

In our example, the safe areas ESA defined in Block 15 are then used forimproving the Segmentation in Block 18. Namely, the safe areas ESA aretaken into account when building maps of the layers (say, SEM-maps) inthe modified box 18A. The obtained map of a specific layer will comprisemap safe areas MSA corresponding to the ESA which were defined for thatspecific layer.

Each MSA may be defined by comparing a specific ESA with the map of thelayer of interest.

The safe areas determined on the expected images and maps of the layers(ESA and MSA) are then used for accurate measurement of overlay betweentwo selected layers. Due to that, the general Block 20 is modified inFIG. 3 and marked 20A (since the OVL measurement is based on the safeareas.)

Examples of using safe areas for overlay measurement will be presentedin the flow chart of FIG. 6 and in a pictorial presentation of FIG. 8.

FIG. 4 illustrates a flowchart of a modified method for overlaymeasurement, which comprises a feedback based on the measurementresults. For example, such a method comprising the feedback may startwith the flowchart 10 described with reference to FIG. 2.

In the example of FIG. 4, the flowchart 10 is shown partially: onlyblocks 18 and 20 are seen.

When an overlay result is calculated (for example, by Block 20 offlowchart 10), the result is compared with a predetermined OVL limit(block 22). If the limit is exceeded, the product (such as asemiconductor wafer) is considered defective. The defective wafer may bediscarded; alternatively, upper layer(s) of the structure may be removedand then deposited again.

If the OVL limit is not exceeded, OVL can be improved in case there areavailable resources to perform it (since additional rounds for improvingaccuracy cost extra time, energy, materials, etc.—block 24). If yes, theInventors suggest improving the overlay by adjusting the expected(design) image of one or both of the two layers which were checked fortheir overlay (block 26). For example, simulation of the expectedimage(s) may be adjusted, and/or the expected images (CAD-images) may beshifted one relatively to another, etc.

The corrected expected image of a specific layer is then fed back to theSegmentation block 18 to improve results of Segmentation. TheSegmentation will be then performed with reference to the updatedexpected image of that layer (at least in box 18.2). Box 27 receivesinformation on the expected layer image from block 14 and block 26, andwill select there-between the most update information for feeding it toblock 18.

Such updated information from block 26 will also be fed to block 20which is responsible for measuring OVL. Owing to that, at the next roundof fabrication, overhead will be re-calculated with reference to thecorrected expected image of that specific layer.

At a specific stage of the process, a decision can be made at block 24that any further improvement of OVL is useless or too expensive.

FIG. 5 illustrates one possible version of measuring overlay based onthe general algorithm defined in block 20 (FIG. 2), optionally using amodified version defined in block 20A (FIG. 3). General algorithm ofblock 20 schematically describes the combined processing of maps andexpected images of any two selected layers, to measure overlaythere-between. Block 20A adds to Block 20 a possibility to perform theprocessing using so-called safe areas.

FIG. 5 schematically illustrates a specified flowchart of OVLcalculation, which comprises:

Box 20.1, where any two layers (called layer 1 and layer 2) are selectedto measure OVL there-between;

Box 20.2, where registration is performed for each of the selectedlayers. Such a per-layer registration comprises aligning of the expectedimage (say, a CAD image or a simulated CAD image) of a specific layeragainst its map (SEM-map). The operation is performed for layer 1 andfor layer 2 based on their respective CAD and SEM images. The per-layerregistration may be facilitated, if performed based on safe areaspreliminarily defined on the expected images (CAD-simulated images) andmaps (SEM-images) of the layer.

Box 20.4 comprises measuring a shift (an x/y vector) of a layer's mapfrom the layer's expected image. For layer 1, the measured vector willbe called “shift 1”. Upon measurement of such a vector for layer 2,“shift 2” is received.

Box 20.6 comprises calculation of overlay (OVL) as a difference betweenthe two shifts obtained at box 20.4.

FIG. 6 shows another version of measuring overlay (OVL) between any twolayers of the multilayer structure. The concept of safe areas isutilized in this version, therefore the algorithm is generally marked as20A. However, the version differs from the one of FIG. 5 by some newoperations performed with the safe areas:

Box 20.1—selecting two layers for the OVL measurement (the same as inFIG. 5).

Box 20.3—calling, from the computer memory, the maps of the two selectedlayers (i.e., segmentation maps of the layers) and ensuring that themaps are arranged in their real mutual positions (as in the structure).

Box 20.5—safe areas are identified on the layer maps. Let for example,safe area 1 (MSA1) is identified on the layer map of layer 1, and safearea 2 (MSA2) on the map of layer 2.

Box 20.7—determining Centers of Gravity (COGs) for the identified safeareas, and measuring vector V1 between the COGs of the two safe areasMSA1 and MSA2. The vector V1 will indicate a “visible overlay” betweenmaps of the layers. In our example, vector V1 is determined for theSegmentation maps of the layers.

Box 20.9—is for obtaining from the computer memory two expected imagesof the layers 1, 2, and for placing these expected images in theirdesigned mutual positions.

20.11—on the expected images of layers 1 and 2, identifying the segmentswhich correspond to the safe areas MSA1, MSA2 of the respective layermaps. These segments are actually the safe areas ESA1, ESA2 of theexpected images.

20.13—finding COGs of these segments (ESA1, ESA2), and measuring vectorV2 between the COGs of ESA1 and ESA2. In our example, V2 is determinedfor the expected images being CAD-images of the layers.

20.15—calculating overlay OVL between the layers 1 and 2 as thedifference between the vectors V1 and V2.

FIG. 7 comprises six parts 7 a-7 f.

Part 7 a is a combined expected image of a 4-layer structure, wherefeatures/elements of each layer are marked by digits 1-4. Each digit (1,2, 3, 4) indicates the number of the layer where the feature is to belocated according to the design. In this example, the expected image isa simulated design image.

Part 7 b shows a schematic pictorial view of the given image (in thisexample, a SEM-image) of the real structure. Spots of the given imagemay be then segmented by referring to the expected image of Part 7 a.

Part 7 c is an expected image of layer 3, where the dark silhouettecorresponds to the diagonal element located on layer 3. A dashed contouraround the diagonal element 3 shows its allowed size deviation(delta-3). Dashed vertical lines show limits of size deviation of avertical element 4 which is located on layer 4. The size deviation ofthe element 4 is marked as delta-4. In reality, element 4 partiallyoccludes element 3.

There is also an arrow “OVL lim” which schematically shows the limit ofoverlay between layer 3 and layer 4.

Part 7 d shows a segmentation map of layer 3, obtained upon segmentationof the given image of Part 7 b. The map of layer 3 (Part 7 d) shows onlythe segments of layer 3 which are seen on the given image. This map oflayer 3 may be used for performing registration with the expected imageof layer 3 (Part 7 c) for further measurement of overlay, for examplebetween layer 3 and layer 4.

Part 7 e schematically shows how expected safe areas ESA may be definedon the expected image of layer 3, if we are to determine mutualpositions (overlay) between layers 3 and 4. ESA of layer 3 are the areaswhich will remain visible in the worst case of offset i.e., when boththe size deviations and the overlay concerning layers 3, 4 take place.

Fig. Part 7 f schematically shows safe areas MSA on the map of layer 3,which, in the worst case, may correspond to the safe areas ESA on theexpected image of that layer.

FIG. 8 presents a pictorial illustration of how to measure overlaybetween two exemplary layers (layer 1 and layer 2), using the concept ofsafe areas. The relevant algorithm is generally described with referenceto FIG. 6.

Let the left-hand portion “A” of FIG. 8 illustrates two expected images(CAD-images) of layers 1 and 2 in their expected position with respectto one another. The right-hand portion “B” shows two segmentation mapsof the layers 1 and 2, in their real position regarding one another.

It should be reminded that areas which are not occluded at a specificlayer, are considered safe areas of that layer.

Let layer 1 is an upper layer, and layer 2 is a lower layer. On layer 1,there is a vertical element marked L1, which is seen as non-occluded. Onlayer 2, there is a diagonal element marked L2. L2 is partially occludedby L1 according to the design (the left-hand diagram “A” of FIG. 8). L2is partially occluded by L1 also on the real, Segmentation maps of thelayers (the right hand portion “B” of FIG. 8), but one may notice thatthe position of L2 is visually shifted due to some deviations of sizesand/or overlay.

Let the darkened segments are safe areas of the element L2. At theportion “B”, the darkened segment of L2 and the whole L1 are mapped safeareas (MSA). At the portion “A”, only the darkened segment of L2suitable to “B” and the whole L1 are ESA.

Now Centers of Gravity (COG) are determined for all the safe areas.

Then, vector V1 is determined for “B” (SEGM-maps of layers 1, 2), as adifference between COGs of the two safe areas MSA located at twodifferent layers 1 and 2.

Analogously, vector V2 is determined for “A” (CAD-images of layers 1,2), as a difference between COGs of the two safe areas ESA located ontwo different layers 1 and 2.

Finally, the overlay between layer 1 and layer 2 is calculated as adifference between the vectors V1 and V2: OVL=V1−V2.

Though the invention has been described with references to specificexamples and drawings, other, modified versions of the method anddifferent implementations of the system might be proposed, which shouldbe considered part of the invention whenever defined by the followingclaims which follow.

1. A method for determining overlay between layers of a multilayerstructure, the method comprising obtaining a given image representingthe multilayer structure, obtaining expected images for layers of themultilayer structure; providing a combined expected image of themultilayer structure as a combination of the expected images of saidlayers; performing registration of the given image against the combinedexpected image; providing segmentation of the given image, therebyproducing a segmented image, and maps of the layers of said multilayeredstructure; determining overlay between any two selected layers of themultilayer structure by processing the maps of the two selected layerstogether with the expected images of said two selected layers.
 2. Themethod according to claim 1, wherein the expected images of the layersand/or the combined expected image are design images which haveundergone simulation.
 3. The method according to claim 1, wherein themultilayer structure is a semiconductor structure 3D IC, the given imageis a SEM-image of the real 3D IC, the expected images are simulateddesign images of the layers, the combined expected image is formed bycombining the simulated design images of the layers taking into accountthe order of the layers and the expected occlusions; the segmentation isthe SEM-image segmentation, thereby producing SEM-maps of the layers,the overlay between any two layers of the 3D IC is determined byprocessing the SEM-maps of the two layers using the expected images ofsaid two layers.
 4. The method according to claim 1, wherein saidsegmentation takes into account the expected images of the layers. 5.The method according to claim 1, comprising enhancing the segmentationby adjusting at least one of the expected images.
 6. The methodaccording to claim 1, comprising determining safe areas both on theexpected images and on the maps of the layers, by taking into accountprobable occlusions and distortions between the expected images and thegiven image.
 7. The method according to claim 6, wherein saiddetermining of overlay between any two selected layers of the multilayerstructure is performed by utilizing the determined safe areas.
 8. Themethod according to claim 1, wherein the determining of overlay betweenany two selected layers of the multilayer structure is performed asfollows: for each of the two selected layers, performing per-layerregistration between the map of the specific layer with the expectedimage of the same specific layer; for each of the two selected layers,measuring a shift of a specific layer by comparing the map of thespecific layer with the expected image of the same specific layer,thereby obtaining two shifts of the two selected layers respectively;obtaining the overlay between said two selected layers as a differencebetween said two shifts.
 9. The method according to claim 8, furthercomprising determining safe areas both on the expected images and on themaps of the layers, by taking into account probable occlusions anddistortions between the expected images and the given image; andperforming said per-layer registration based on the safe areas.
 10. Themethod according to claim 6, wherein said determining of overlay betweenany two selected layers of the multilayer structure is performed asfollows: choosing two safe areas of pixels (MSA) respectively appearingin the two selected maps of the two selected layers; allocating twosegments (ESA) respectively appearing in the two expected images of thetwo selected layers, said two segments ESA respectively corresponding tosaid two safe areas MSA of pixels; determining a vector of overlayS_(img) for the two chosen safe areas MSA by calculating a differencebetween Centers of Gravity (COG) thereof; determining a vector ofoverlay S_(exp) for the two segments ESA by calculating a differencebetween Centers of Gravity (COG) thereof; determining overlay betweenthe selected two layers by calculating a difference between S_(img) andS_(exp).
 11. The method according to claim 1, using feedback about thedetermined overlay for correcting the segmentation.
 12. The methodaccording to claim 11, comprising re-determining of overlay using thecorrected segmentation.
 13. The method according to claim 1, comprisingdetermining of overlay for each layer of the multilayered structure,with respect to any of the remaining layers.
 14. A system for imageprocessing, including a memory being configured to store a given imageof a multilayer structure and expected images of layers of themultilayer structure, and a processor configured: for generating acombined expected image, for processing the given image with thecombined expected image so as to perform registration there-between, forperforming segmentation of the given image into maps of thecorresponding multiple layers of the structure, and for performingoverlay measurement between any two layers of the structure, based onthe maps and suitable expected images of said two layers.
 15. A computersoftware product, including a non-transitory computer-readable medium inwhich program instructions are stored, which instructions, when read bya computer, cause the computer to perform the method according to claim1.